Recently, an information transmitting system using a glass fiber for optical transmisstion (hereinafter referred to as optical fiber) has greatly been developed. Optical fibers are characterized by no fear of short circuit, spark, etc., freedom from electromagnetic distrubances, easiness in lightening or thinning, and the like, as compared with usual electric wires. Therefore, optical fibers have recently been employed in various applications as compared with the conventional electric wires. However, since most of optical fibers which are presently used comprise a glass fiber composed of a core and a cladding, a primary coating layer made of a silicone resin and a secondary coating layer made of a nylon resin as disclosed in, e.g., U.S. Pat. No. 3,980,390, and the secondary coating layer is melted in a high temperature atmosphere, e.g., at 200.degree. C., they cannot withstand use in such a high temperature atmosphere.
Known methods for improving heat resistance of polymers, for example, polyethylene, include chemical crosslinking using organic peroxides or crosslinking upon radiation to obtain crosslinked polyethylene which does not undergo substantial thermal deformation at temperatures above the melting point of polyethylene. However, when chemical crosslinking is adopted to the secondary coating layer of optical fibers, heating under pressure involved therein causes internal distortion and the like, thereby resulting in increase of transmission loss. Therefore, this technique cannot be applied to optical fibers. Further, if the secondary coating layer is crosslinked by irradiation, an increase of transmission loss occurs at a very low radiation dose, e.g., 20 rads, as apparently shown in FIG. 1 wherein .gamma. rays are used. Therefore, it appears that radiation having great transmittance, such as .gamma. rays, cannot be utilized for crosslinking of the secondary coating layer of optical fibers.